Method for driving electro-optical display device

ABSTRACT

A matrix of display elements are driven on a time sharing basis to switch between two optically distinct states in response to a pulse train applied to the selected display elements. The pulse train includes display control pulses having a duration and electric field effective to set the display elements to the desired optically distinct state, does not allow, after the application of the display control pulse, existence of a pulse which would change the optical state, and also includes a plurality of pulses having the same pulse shapes in different polarities. AC pulses which hold the optical state are applied to the display elements after the application of the display control pulse. The AC pulses are obtained by superposing high frequency AC pulses to the AC pulses having an amplitude lower than the display control pulse.

This is a continuation of application Ser. No. 887,412 filed May 20,1992 now abandoned, which is a continuation of application Ser. No.338,467 filed Apr. 14, 1989, now abandoned which is a continuation ofapplication Ser. No. 847,187 filed Apr. 2, 1986, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method for driving an electro-opticaldisplay device such as ferroelectric liquid crystal device, etc.

Recently, the ferroelectric liquid crystal is drawing attentions ofresearchers as a successor of the TN type liquid crystal and a displaydevice utilizing such a ferroelectric liquid crystal is currentlyresearched.

The display mode of ferroelectric liquid crystal is categorized into thecomplex refraction type display mode and guest host type display mode.In the case of driving a display device of such modes, the drivingmethod which has been employed for the conventional TN type liquidcrystal cannot be used because the display condition (brightness ofdisplay) is controlled by changing the direction of applied electricfield, unlike the conventional TN type liquid crystal. Namely, when avoltage, although it may be low level, is continuously applied even inone direction, the ferroelectric liquid crystal may at last respond tosuch voltage. Therefore, a special driving method is necessary for suchdisplay mode.

The driving methods which have been developed so far sometimes allow,during time-shared non-selected period, repeated application of smallamplitude pulses having the polarity opposite to that of a pulse fordisplay, and trigger drop of contrast in case that a number of digits tobe displayed increases.

Moreover, a voltage to be applied is not a perfect AC voltage and avoltage of one polarity is applied longer than a voltage of the otherpolarity. Accordingly, there arises a problem that if a device is drivenfor a long period of time, the transparent electrode for display may beblackened or double color pigment may be discolored or liquid crystalmay by deteriorated.

Since the conventional driving methods are followed by such problems,these methods cannot be considered as the definitive method andtherefore advent of the optimum driving method has long been expected.

Moreover, it has been supposed to be difficult to generate theintermediate tone in display for the ferroelectric liquid crystal and adriving method for generating the gradation has not yet been developed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for drivingan electro-optical display device which prevents even after a longperiod of driving that the transparent electrode is blackened, doublecolor pigment is discolored and liquid crystal is deteriorated.

It is another object of the present invention to provide a method fordriving an electro-optical display device which does not bring aboutdrop of contrast even in case a number of digits increases.

It is further object of the present invention to provide a method fordriving the electro-optical display device which can generate theintermediate tone of display.

It is still further object of the present invention to provide a methodfor driving an electro-optical display device which shortens the periodof time-sharing driving and remarkably increases a number of digits fordisplay.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a display device; and FIGS. 2-25respectively show voltage waveforms for driving the display device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 and FIG. 2, a selection circuit SE generates a selectionsignal S₁ (FIG. 2) which is sequentially supplied to a group of rowelectrodes L₁ -L₇, on the time sharing basis to select one of the rowelectrodes L₁ -L₇ and also generates a non-selection signal NS₁ which issupplied to said one of the row electrodes L₁ -L₇ while the selectionsignal S₁ is not supplied thereto. The selection signal S₁ has awaveform composed of voltages V, 2 v (desirably, V/2≧v), while thenon-selection signal NS₁ has another waveform composed of voltages V, v.

Meanwhile, a drive control circuit DR generates a response or drivingsignal D₁ and a reverse response or reverse driving signal RD₁ havingdifferent waveforms from each other as shown in FIG. 2 and supplies suchsignals to a group of columns electrodes R1-R5. Namely, it supplies theresponse signal D₁ to a column electrode connected to a response displayor picture element and the reverse response signal RD₁ to another columnelectrode connected to a reverse response display element.

With supply of these signals R₁ and RD₁, a pulse group or train P₁ isapplied to the response display element which is to be set to one of twodifferent response conditions, while a pulse group P₂ is applied to thereverse response display element which is to be set to the other of twodifferent response conditions. In the pulse group P₁, pulse shapes ofwhich are determined according to the waveform difference between theselection signal S₁ and response signal D₁ to the response displayelement, a pulse (V-2 v) is first applied to the response displayelements but the liquid crystal does not respond to it. Then, theresponse display element; once reversely responds to the next reverseresponse pulse P₁₁ having the negative polarity such that the responsedisplay element is preset to the other response condition. Since asucceeding display control pulse P₁₂ having the positive polarity isapplied next to the response display element, the liquid crystal thereofis saturated by the pulse P₁₂ such that the response display element isset to said one response condition. Thereafter, a pulse having a voltageof -(V-2 v) is applied but the liquid crystal does not respond to thisvoltage and does not switch to the other response condition. In the caseof this pulse group P₁, a number of pulses in different polarities andpulse shapes of such pulses are equal, thereby forming perfect AC pulseshaveng an average voltage of zero.

After the application of the pulse group P₁, an AC pulse train A₁ or A₂is applied by the non-selection signal NS₁, holding the responsecondition. Namely, since the AC pulse train A₁, A₂ are composed ofpulses in which are determined according the waveform difference betweeneither of response signals D₁ and RD₁ and the non-selection signal NS₁and which are the same in the pulse shape but are different only in thepolarity, even when these pulses are repeatedly applied, the liquidcrystal is hold under said one response condition.

On the other hand, in the pulse group P₂, pulse shapes of which aredetermined according to the waveform difference between the selectionsignal S₁ and reverse response signal RD₁ as opposed; to the pulse groupP₁, after the proceeding response pulse P₂₁ having the picture polarityis applied to the reverse response display element to preset the same tosaid one display condition, a succeeding display control pulse P₂₂having the negative polarity is applied for setting the liquid crystalto the other response condition. Thereafter, pulses of voltages (V-2 v),-(V-2 v) are applied and thereby the other response condition ismaintained. When the non-selection signal NS₁ is being supplied, the ACpulse train A₁ or A₂ is applied and thereby the other response conditionis held.

As explained above, the pulse groups P₁, P₂ and AC pulses A₁, A₂ are alla perfect AC pulse train which has a number of pulses different in thepolarity to average out the voltage to zero. Therefore, the transparentrow and column electrodes are not blackened, the liquid crystal is notdeteriorated and double color pigment is not discolored.

For instance, in the case of ferroelectric liquid crystal cell in thethickness of 10 μm, the saturated responsive condition and saturatedreverse responsive condition or the two optically different states canbe obtained by setting a voltage V=10 volts and setting a pulse durationof the display control pulse to 250 μs.

FIG. 3 and FIG. 4 indicate other examples of respective signals andthese signals ensure the similar drive as in the case of FIG. 2.

In FIG. 5 and FIG. 6, the duration of a pulse applied to the displayelement during non-selection period can be set to 1/2 or less of theduration of the display control pulse by changing the waveform ofnon-selection signal. Namely, waveforms in FIG. 5 and FIG. 6 arerespectively similar to those modified from the non-selection signals ofthe examples in FIG. 3 and FIG. 4 and the AC pulses A₃ or A₄ and A₅ orA₆ of small duration are applied as shown in the figure during thenon-selection period. Therefore, as compared with the examples of FIG. 2to FIG. 4, the response condition or reverse response condition ofdisplay element can be acquired more reliably and an allowance fortemperature charge and variation of cell thickness becomes large.

In the examples of FIG. 2 to FIG. 6, when an AC pulse is applied,following the pulse group including the display control pulse, thepulses of the same polarity are sometimes applied continuously. Namely,in FIG. 2, for example, since the polarity of the last pulse of thepulse groups P₁, P₂ is the same as that of the first pulse of the ACpulse A₁, the pulse of the same polarity are applied in succession whenthe pulse group P₁ or P₂ is switched to the AC pulse A₁. It is a factorfor reducing the margin for temperature change and cell thicknessvariation.

The examples shown in FIG. 7 to FIG. 9 are proposed for eliminating suchdisadvantage. In FIG. 7, the polarity of the last pulses of pulse groupsP₃, P₄ is the same and it is the reverse the polarity to that of thefirst pulses of the AC pulses A₇, A₈. Thereby, the pulses of the samepolarity are never applied successively and thereby the margin becomeslarge.

In an example of FIG. 8, the last pulses of pulse groups P₅, P₆ arereversed in polarity from the first pulses of AC pulses A₉, A₁₀, byadding a narrow pulse having a voltage of V/2 or voltage 0 to theleading part or trailing part of each signal.

FIG. 9 shows the waveforms wherein a narrow pulse in voltage of v orvoltage 0 is adding to the leading part or trailing part of each signalshown in the example of FIG. 2. Thereby, the last pulses of the pulsegroups P₇, P₈ are reversed in polarity from the first pulses of the ACpulses A₁₁, A₁₂.

Moreover, in the examples of FIG. 7 to FIG. 9, the waveforms and numberof pulses are the same in the pulses in different polarities and perfectAC driving can be realized.

The same effect can also be obtained by adding the pulse similar to thatin FIG. 9 to the examples of FIG. 3 to FIG. 6.

An example of obtaining the intermediate tone of display by givinggradation to the display. FIG. 10 is an example for generatingintermediate tone by controlling the waveforms of response signals inthe example of FIG. 9 in accordance with the gradation. Namely, a partof the first voltage V of the response signal D₂ is removed to form avoltage (V-v) and simultaneously a voltage v which has the same durationas that removed above is added to the next voltage V. Thereby, a part ofthe display control pulse P₉₁ of the pulse group P₉ is removed and thepulse P92 which has the reverse polarity and the same waveform to saidpulse is also removed. Therefore, the intermediate tone can be freelygenerated by controlling the width of such voltage pulse to be removedin accordance with the gradation. Moreover, the perfect AC driving canbe realized using the pulses having the same waveform and numbers butare different in the polarities.

FIG. 11 shows an example for obtaining the intermediate tone in theexample of FIG. 3. In this example, a voltage V of the response signalD₃ is set to (V-a) (a is a voltage corresponding to the gradation) andthe next voltage 0 is changed to a voltage (a). Thereby, the voltage ofthe display control pulse P₁₀₁ of pulse group P₁₀ drops to (V-a),generating the intermediate tone. In this case, the pulse P₁₀₂ which isin the reverse polarity and same waveform as the display control pulseP₁₀₁ also drops by only a voltage (a), maintaining the perfect ACdriving. Moreover, since the first two pulses of the AC pulse A₁₃ dropsonly by a voltage (a), the margin for temperature change improves.Moreover, in this example, the circuit structure can be simplifiedbecause it is enough to change only a voltage.

The intermediate tone can also be generated by this method even in thecase of examples of FIG. 3 to FIG. 7. In other words, this method can beadapted only when the display control pulse for obtaining the responsecondition, a pulse which is different therefrom only in the polarity,the display control pulse for obtaining the reverse response conditionand a pulse which is different therefrom only in the polarity are,respectively, shifted in time. In other words, as in the case of exampleshown in FIG. 8, this method cannot be adopted when the display controlpulse P₅₁ of the pulse group P₅ is overlapped in timing on the pulse P₆₁of the pulse group P₆.

However, above two kinds of methods for generating the intermediate tonehave difficulty in displaying dynamic images. For example, in theexample shown in FIG. 11, when the pulse group P₁₀ is applied to thedisplay elements in the saturated response condition, the liquid crystalis once set to the unsaturated reverse response condition by the pulseP₁₀₂ and is then set to the unsaturated response condition by the pulseP₁₀₁ from the condition. However, when the pulse group P₁₀ is firstapplied to the display elements in the saturated reverse responsecondition, the saturated reverse response condition is maintained by thepulse group P₁₀₂, and since the pulse P₁₀₁ is applied in this condition,the liquid crystal is set to the unsaturated response condition which isdifferent from that described above.

Therefore, the final response condition depends on the precedingcondition and it is difficult to obtain the desired response condition.

In the following example, the desired intermediate tone can begenerated, irrespective of the preceding condition. In FIG. 12, theselection signal is sequentially supplied to the electrodes L₁ -L₇ likethe above examples and the non-selection signal is also supplied duringthe non-selection period. Meanwhile, the control signal C₁ is suppliedto the electrodes R₁ -R₅. The control signal C₁ is composed of voltages0, V, (v-a) and (a) and the voltage (a) is changed in accordance withthe gradation. After the response pulse P₁₁₁ is applied in accordancewith a voltage difference between the selection signal and controlsignal C₁, the reverse response pulse P₁₁₂ is supplied. Therefore, theliquid crystal is once set to the saturated reverse response condition,irrespective of the preceding condition, and thereby initialization iscarried out. Accordingly, the desired response condition can be obtainedby the display control pulse P₁₁₃ of voltage (V-a).

When the intermediate tone is thus obtained, such condition is kept bythe AC pulse A₁₄.

For instance, when a voltage (a) is set to 0, the pulse P₁₁₃ becomesvoltage V to set the saturated response condition. When the voltage of(a) is set to V, the pulse P₁₁₃ becomes 0 and the saturated reverseresponse condition is held by the pulse P₁₁₂.

As explained above, the liquid crystal is once set to the saturatedreverse response condition before the pulse for generating theintermediate tone and therefore stabilized intermediate tone can beobtained even in the display of quickly moving images.

FIG. 13 shows the polarities of voltages applied which are reversed fromthose in FIG. 12.

FIG. 14 and FIG. 15 show the examples for generating the intermediatetone by adjusting a pulse duration. The control signal C₂ in FIG. 14 isa modification of the control signal C₁ in FIG. 12. In this case, thedurations of voltages V and (V-v) are controlled in accordance withgradation. Thereby, the pulse P₁₂₁ and P₁₂₂ of pulse group P₁₂ becomethe stepwise wave of voltages V and v and duration of voltage V changesin accordance with gradation, generating the intermediate tone. The ACpulse A₁₅ also becomes the stepwise wave and since this pulse is same asthe above pulse and is different only in the polarity, aboveintermediate tone can be held.

In FIG. 15, the pulse group P₁₃ in the opposite polarity to the pulse inFIG. 14 is applied for display.

Explained hereunder is an example where display is once initialized atthe timing before supplying the selection signal and thereafter thecondition is changed. In FIG. 16, the selection signal S₂ consisting ofvoltages -V and V is sequentially supplied to the electrodes L₁ -L₇ inFIG. 1, and the reset signal RS consisting of voltages V and -V issupplied at the preceding timing. During non-selection period, thenon-selection signal NS₂ consisting of voltages v and v (desirably,V/4≦v≦V/2) is supplied.

Meanwhile, the response signal D₄ in voltage 0 or the reverse responsesignal RD₂ consisting of voltages -2V and 2V is supplied to theelectrodes R₁ -R₅.

First, the pulse group P₁₄ or P₁₅ is applied by the supply of the resetsignal RS and thereby the liquid crystal is once reset to the saturatedreverse response condition. Moreover, it is then set to the responsecondition by applying the pulse group P₁₆ with the selection signal S₂and response signal D₄, while it is set to the reverse responsecondition by applying the pulse group P₁₇ with the selection signal S₂and reverse response signal RD₂. The pulse group P₁₇ is used for holdingthe saturated reverse response condition by the pulse group P₁₅.

When the non-selection signal NS₂ is supplied, the AC pulse A₁₆ or A₁₇is applied, maintaining the response condition or reverse responsecondition.

According to this example, each signal supply period becomes 1/2 of thatin the previous examples example and therefore a number of digits to bescanned in the same period can also be doubled, realizing themulti-digit driving. In other words, the single scanning time can becurtailed to 1/2 for the same number of digits, crosstalk can be reducedand contrast can be improved.

FIG. 17 and FIG. 18 show the examples for generating the intermediatetone utilizing the example of FIG. 16. In FIG. 17, the reset signal,selection signal and non-selection signal are the same as those in FIG.16 and a voltage (a) of control signal C₂ to be supplied to theelectrodes R₁ -R₅ is controlled or modulated in accordance with thegradation. The pulse group P₁₈ of voltages (V+a) and -(V+a) is appliedaccording to the reset signal RS and control signal C₂ and the liquidcrystal is reset to the saturated reverse response condition.Thereafter, the pulse group P₁₉ of voltages -(V-a) and (V-a) is appliedin accordance with the selection signal S₂ and control signal C₂ andthereby desired response condition can be obtained. The AC pulse A₁₈ ofvoltages -(v-a) and (v-a) is applied in accordance with thenon-selection signal NS₂ and control signal C₂ and thereby the responsecondition can be held.

In FIG. 18, the gradation is obtained by adjusting pulse duration anddurations of voltages 2 v, -2 v of the control signal C₃ are adjusted inaccordance with the gradation. Thereby, as in the case described above,the liquid crystal is reset to the saturated reverse response conditionby the pulse group P₂₀. Thereafter, it is set to the desiredintermediate response condition by the pulse group P₂₁ and this responsecondition is held by the AC pulse A₁₉. The pulse group P₂₁ is capable ofgenerating the desired intermediate tone since the durations of voltagesV and -V changes in accordance with the gradation.

In the examples of FIG. 17 and FIG. 18, the liquid crystal is reset tothe saturated reverse response condition before rewriting of display andtherefore stable intermediate tone can be generated irrespective of thepreceding response condition.

Explained hereunder is an example where a high frequency AC pulse issuperposed on the non-selection signal.

In FIG. 19, while the selection signal S₃ is not supplied, thenon-selection signal NS₃ is generated. The selection signal S₃ iscomposed of voltages -(V-2 v), (V-v), -V and the non-selection signalNS₃ is composed of the AC pulses of voltages 0, H.

The response signal D₅ or reverse response signal RD₃ is supplied to theother electrodes R₁ -R₅. Namely, the response signal D₅ is supplied tothe column electrode of response display element, while the reverseresponse signal RD₃ is applied to the other column electrode of thereverse response display element.

With supply of above signals, the pulse group P₂₂ is applied to theresponse display element, while the pulse group P₂₃ is applied to thereverse response display element. In the case of pulse group P₂₂, apulse of voltage (V-2 v) is applied but the liquid crystal does notrespond to it. When next reverse response pulse P₂₂₁ is applied, theliquid crystal once reversely responds to it. But, since the displaycontrol pulse P₂₂₂ is applied next, the liquid crystal is set to thesaturated response condition. Thereafter, a pulse of voltage -(V-2 v) isapplied, but the liquid crystal does not respond to this pulse and isnot set to the reverse response condition. In the pulse group P₂₂, thepulses are the same in the number and waveform but are different in thepolarities and the perfect AC pulses are obtained.

After the pulse group P₂₂ is applied, an AC pulse A₂₀ or A₂₁ which isobtained by superposing a high frequency AC pulse to a pulse +v which islower than the response pulse V is applied and thereby the responsecondition can be held. Namely, since the AC pulses A₂₀, A₂₁ are composedof the narrow AC pulses which are the same in the waveform but differentonly in the polarity, the liquid crystal is held at the responsecondition even when such pulse is applied repeatedly. Particularly inthe case of ferroelectric liquid crystal having negative dielectricanisotropy, a stably holding force can be obtained because the highfrequency AC pulse causes the liquid crystal molecules to be arranged inparallel to the electrode substrate.

Meanwhile, in the case of pulse group P₂₃, after the pulse of ±(V-2 v)to which the liquid crystal does not respond is applied, the responsepulse P₂₃₁ is applied, on the contrary to the pulse group P₂₂. Next, thedisplay control pulse P₂₃₂ is applied for setting the reverse responsecondition. While the non-selection signal NS₃ is supplied, the AC pulseA₂₀ or A₂₁ is applied and the reverse response condition is held.

As described above, since the pulse groups P₂₂, P₂₃ and the AC pulsesA₂₀, A₂₁ are all the same in the number of pulses and waveforms indifferent polarities, problems such as blackening of transparentelectrodes, deterioration of liquid crystal and discoloration ofdouble-color pigment can be eliminated.

For instance, the saturated response condition or saturated reverseresponse condition of the ferroelectric liquid crystal in the thicknessof 10 μm can be obtained by setting a voltage V to 10 volt and durationof display control pulse to 250 μs.

It is better to set the frequency of high frequency AC pulse to twotimes or more (desirably, 4 times or more of integer times) as that ofthe response pulse frequency and a pulse amplitude H is adequatelydetermined so that the response condition is kept stably from a relationwith the dielectric anisotropy of the ferroelectric liquid crystal, butit is usually desirable that the pulse amplitude H is about the responsepulse amplitude V or less.

FIG. 20, FIG. 21 FIG. 22 and FIG. 23 indicate other examples ofrespective signal waveforms and each example realizes the drivingsimilar to that in FIG. 19.

Explained next is the case where the display is once reset at the timingbefore supply of selection signal and thereafter condition is changed.In FIG. 23, the selection signal S₄ consisting of voltages of V-v and-(V-v) is sequentially supplied to the electrodes L₁ -L₇ shown in FIG. 1and the reset signal RS consisting of voltages of -(V+v) and V+v issupplied at the previous timing. During non-selection period, thenon-selection signal NS₄ consisting of voltages of +H is supplied.

On the other hand, the response signal D₆ of voltages -v and v or thereverse response signal RD₄ of voltages of v and -v is supplied to theelectrodes R₁ -R₅.

First, the pulse group P₂₄ or P₂₅ is applied by the supply of the resetsignal RS and thereby the liquid crystal is once reset to the saturatedreverse response condition. Moreover, it is set to the responsecondition by applying the pulse group P₂₆ according to the selectionsignal S₄ and response signal D₆, and can also be set to the reverseresponse condition by applying the pulse group P₂₇ according to theselection signal S₄ and reverse response signal RD4. The pulse group P₂₇holds the saturated reverse response condition set by the pulse groupP₂₄ or P₂₅.

When the non-selection signal NS₄ is supplied, the AC pulse A₂₂ or A₂₃is applied and thereby the response condition or reverse responsecondition is held.

According to this example, since the supply period of each signalbecomes 1/2 or 2/3 of that in examples described above, a number ofdigits to be scanned in the same period can be set to two times or 1.5times, making possible the multi-digit drive. In other words, when thenumber of digits is the same, single scanning period can be reduced to1/2 or 2/3 and thereby the crosstalk can be reduced and contrast can beimproved.

An example of displaying the intermediate tone is explained. FIG. 24 andFIG. 25 indicate the examples for generating the intermediate tone byutilizing the examples of FIG. 22 and FIG. 23. In FIG. 24 and FIG. 25,the reset signal, selection signal and non-selection signal are the sameas those in FIG. 22 and FIG. 23, and a voltage (a) of the controlsignals C₄ and C₅ to be supplied to the electrodes R₁ -R₅ is controlledin accordance with the gradation. In FIG. 24, the response pulse P₂₈₁and reverse response pulse P₂₈₂ are first applied by a voltagedifference between the selection signal and control signal C₄, onceinitializing the liquid crystal to the saturated reverse responsecondition. Thereafter, the saturated reverse response condition is heldby the unsaturated reverse response pulse P₂₈₃ and finally theunsaturated response pulse P₂₈₄ is applied and the intermediate tone isdisplayed.

In FIG. 25, the condition is once reset to the saturated reverseresponse condition by the pulse P₂₉ according to the reset signal RS andcontrol signal C₅, thereafter the saturated reverse response conditionis held by unsaturated reverse response pulse P₃₀₂ in accordance withvoltage difference between the selection signal S₄ and control signal C₅and the intermediate tone is displayed by the unsaturated response pulseP₃₀₁. Thereafter, a high frequency AC pulse is applied according to thenon-selection signal and control signal and said response condition canbe held.

As the pulse for displaying intermediate tone, this intermediate tonecan be displayed not only by modulation of voltage (a) of the controlsignal but also by the pulse width modulation. In any case, it isimportant that the liquid crystal is once reset to the saturated reverseresponse condition before the pulse for displaying the intermediatetone. If the pulse for displaying the intermediate tone is onlysupplied, the response condition is affected by the display conditionbefore the application of pulse and thereby stable display ofintermediate tone is impossible. For example, in case only theunsaturated reverse response pulse and unsaturated response pulse areapplied to the picture elements in the saturated response condition inorder to display the intermediate tone, the picture elements set in theunsaturated reverse response condition by the unsaturated reverseresponse pulse is returned to the saturated response condition by theopposite unsaturated response pulse having the same waveform as the nextunsaturated reverse response pulse and thereby unsaturated responsecondition (intermediate tone) cannot be displayed in some cases.

However, in the examples of FIG. 24 and FIG. 25, since the liquidcrystal is initialized to the saturated reverse response conditionbefore rewriting of display, the intermediate tone can be displayedstably irrespective of the preceding response condition.

In the above explanation, the term "response" is used for a positivevoltage while "reverse response" for a negative voltage, but "reverseresponse" is used for a positive voltage while "response" for a positivevoltage because the "response" and "reverse response" are relative tofront side and rear side of the display.

The signals supplied to the electrodes are not limited only to thosedescribed above and these signals can be modified and moreover a biasvoltage can be added adequately when required.

In addition to the drive of ferroelectric liquid crystal device, thepresent invention can be adapted to any type of devices which controlsthe display condition in accordance with the direction of appliedelectric field and which changes the response rate in accordance withfield intensity and pulse duration, such as a display device utilizingferroelectric electro-optical modulation material such as PLZT, etc. anda display device (EPID) utilizing electrophoresis.

It is certain that color display can be realized by driving a displaydevice comprising a color filter for three colors of red, green and blueby the method of the present invention.

According to the present invention, in the pulse group to be applied tothe display elements, the pulses in different polarities are the same inthe waveform and numbers and therefore, the transparent electrodes arenot blackened, the double color pigment is not discolored and liquidcrystal is not deteriorated even after the driving for a long period oftime. Moreover, since the AC pulse is applied for holding the responsecondition during non-selection period, contrast is not lowered even incase a number of display digits increases.

Moreover, it is also possible to display the intermediate tone which hasbeen considered difficult and the application range can be expandedremarkably.

In addition, the duration of a period of signals to be supplied torespective electrodes can be very shortened by resetting the display atthe timing before the application of selection signal, many digits canbe scanned within a short period and a number of display digits can beincreased remarkably. In other words, when a number of digits is thesame, the rewriting time of display can be much curtailed, crosstalk canbe eliminated and contrast can also be improved.

Further, the intermediate tone can easily be displayed stably, showingdistinctive effect in wide areas such as display of television pictures.

Moreover, the response pulse and reverse response pulse do not include ahigh frequency component, therefore, driving is possible only with a lowvoltage. During non-selection period, since an AC pulse is superposedwith a high frequency AC pulse for holding the response condition,contrast is not lowered even when a number of digits increases.

Meanwhile, in case the ferroelectric liquid crystal having negativedielectric anisotropy is used, the high frequency AC pulse componentcauses the liquid crystal molecules to be arranged in parallel to theelectrode substrate. Accordingly, more stable holding force can beobtained and high contrast display can be realized without crosstalk.Moreover, since the low frequency bias pulse is lower than the responsepulse during the non-selection period, the high frequency AC pulse isnot required to have so high amplitude and a low voltage drive ispossible, as a whole.

What is claimed is:
 1. A multiplexed driving method of an opticalswitching element employing ferroelectric liquid crystal with a negativedielectric anistropy including a plurality of signal electrodes and ofcommon signal electrodes arranged in matrix and a ferroelectric liquidcrystal layer disposed between the signal electrodes and the commonsignal electrodes so as to constitute a plurality of pixels at therespective facing portions of the signal electrodes and the commonsignal electrodes comprises:a step of applying a multi polar pulse witha predetermined amplitude and duration on one of the common signalelectrodes to select pixels covered thereby for information writingduring a first information writing period, applying either a lighttransmitting signal voltage of first polarity or a light cutoff signalvoltage of second polarity on the respective signal electrodes dependingon a first set of input signals during the first information writingperiod, and applying an AC voltage with a predetermined amplitude andfrequency on the other common signal electrodes during the firstinformation writing period, whereby either first or second informationwriting voltages formed in combination with the multi polar pulse andthe light transmitting signal voltage and the light cutoff signalvoltage and being enough to determine one of the light transmitting andcutoff statuses are applied on the respective first selected pixels, andeither first or second status holding voltages formed in combinationwith the AC voltage and the light transmitting signal voltage and thelight cutoff signal voltage and being enough to hold the previouslywritten statuses are applied on the respective first non-selectedpixels; and a step of applying the multi polar pulse with thepredetermined amplitude and duration on another one of the common signalelectrodes to select pixels covered thereby for information writingduring a second information writing period, applying either the lighttransmitting signal voltage of first polarity or the light cutoff signalvoltage of second polarity on the respective signal electrodes dependingon a second set of input signals during the second information writingperiod and applying the AC voltage with the predetermined amplitude andfrequency on the common signal electrodes other than the one appliedwith the multi polar pulse during the second information writingperiods, whereby either the first or second information writing voltagesformed in combination with the multi polar pulse and the lighttransmitting signal voltage and the light cutoff signal voltage andbeing enough to determine one of the light transmitting and cutoffstatuses are applied on the respective second selected pixels, andeither the first or second status holding voltages formed in combinationwith the AC voltage and the light transmitting signal voltage and thelight cutoff signal voltage and being enough to hold the previouslywritten statuses are applied on the respective second non-selectedpixels.
 2. The multiplexed driving method of an optical switchingelement employing ferroelectric liquid crystal with negative dielectricanistropy according to claim 1 wherein the first and second statusholding voltages include a bias voltage determined by either the lighttransmitting signal voltage or the light cutoff signal voltage having asmaller amplitude than those of the first and second information writingvoltages.
 3. The multiplexed driving method of an optical switchingelement employing ferroelectric liquid crystal with negative dielectricanistropy according to claim 2 wherein the bias voltage is a DC biasvoltage.
 4. The multiplexed driving method of an optical switchingelement employing ferroelectric liquid crystal with negative dielectricanistropy according to claim 2 wherein the bias voltage is a AC biasvoltage.
 5. The multiplexed driving method of an optical switchingelement employing ferroelectric liquid crystal with negative dielectricanistropy according to claim 1 wherein the first and second informationwriting voltages include a first pulse having an enough amplitude andduration to determine the orientation of the ferroelectric liquidcrystal molecules in the selected pixels.
 6. The multiplexed drivingmethod of an optical switching element employing ferroelectric liquidcrystal with negative dielectric anistropy according to claim 5 whereinthe first and second information writing voltages include a second pulsehaving opposite polarity from that of the first pulse and of anamplitude not to cause reorientation of the ferroelectric liquid crystalmolecules in the selected pixels.
 7. The multiplexed driving method ofan optical switching element employing ferroelectric liquid crystal withnegative dielectric anistropy according to claim 1 wherein the lighttransmitting signal voltage of first polarity is a first four polarpulse of one sense of polarity with a predetermined amplitude, the lightcutoff signal voltage of second polarity is a second four polar pulse ofthe other sense of polarity with the same predetermined amplitude asthat of the first four polar pulse and the multi polar pulse is a threepolar pulse with an amplitude two times larger than those of the firstand second four polar pulses.
 8. The multiplexed driving method of anoptical switching element employing ferroelectric liquid crystal withnegative dielectric anistropy according to claim 1 further comprises astep of initializing all of the pixels before the information writingthereon by applying an initializing voltage on all of the pixels so thatferroelectric liquid crystal molecules in all of the pixels orient tosubstantially a same direction.
 9. A method for driving anelectro-optical device having optical elements disposed at theintersections between two sets of spaced-apart and intersectingelectrodes for applying an electric field to each optical elementtherebetween, wherein each optical element comprises electro-opticalmodulation material switchable into first and second response states inresponse to a control pulse of respective first and second polaritiesand a given amplitude and duration, the method comprising the stepsof:switching each optical element into a desired one of the first andsecond response states; and thereafter holding each optical element inthe desired response state until the optical element is to be switchedagain; wherein the steps of switching and holding comprise sequentiallysupplying a selection signal to one of the first set of electrodes whilesupplying a non-selection signal to other electrodes of the first set toselect one set of optical elements, and selectively supplying to eachelectrode of the second set of electrodes during the supplying of theselection and non-selection signals one of first and second differentresponse signals corresponding to the first and second response statesrespectively to switch each optical element in the selected set to thedesired response state, wherein the selection signal, non-selectionsignal, first response signal and second response signal comprise pulsesconfigured in polarity, duration and amplitude to effect as a differencetherebetween an AC pulse train applied to each optical element which issymmetrical about a zero voltage axis with regard to total pulseamplitude and duration and includes, during the supplying of theselection signal, a control pulse effective to switch the opticalelement into the desired response state and during the supplying of thenon-selection signals, pulses which are ineffective to switch theoptical element from the desired response state.
 10. The methodaccording to claim 9, further comprising stabilizing the response statesof the electro-optical modulation material by high frequency AC pulseson the pulse train, wherein the high frequency pulses have a higherfrequency than that of the control pulses.
 11. A method for driving anelectro-optical device having optical elements disposed at theintersections between two sets of spaced-apart and intersectingelectrodes for applying an electric field to each optical elementtherebetween, wherein each optical element comprises electro-opticalmodulation material switchable into first and second response states inresponse to a control pulse of respective first and second polaritiesand a given amplitude and duration, the method comprising the stepsof:switching each optical element into a desired gradation of one of thefirst and second response states; and thereafter holding each opticalelement in the desired response state until the optical element is to beswitched again; wherein the steps of switching and holding comprisesequentially supplying a selection signal to one of the first set ofelectrodes while supplying a non-selection signal to other electrodes ofthe first set to select one set of optical elements, and selectivelysupplying to each electrode of the second set of electrodes during thesupplying of the selection and non-selection signals one of first andsecond different response signals corresponding to the first and secondresponse states respectively to switch each optical element in theselected set to the desired response state, wherein the selectionsignal, non-selection signal, first response signal and second responsesignal comprise pulses configured in polarity, duration and amplitude toeffect as a difference therebetween an AC pulse train applied to eachoptical element which is symmetrical about a zero voltage axis withregard to total pulse amplitude and duration and includes, during thesupplying of the selection signal, a control pulse having an adjustedeffective voltage value effective to switch the optical element into thedesired gradation of the response state and, during the supplying of thenon-selection signals, pulses which are ineffective to switch theoptical element from the desired response state.
 12. The methodaccording to claim 11, further comprising stabilizing the responsestates of the electro-optical modulation material by high frequency ACpulses on the pulse train, wherein the high frequency pulses have ahigher frequency than that of the control pulses.
 13. A method fordriving an electro-optical device having optical elements disposed atthe intersections between two sets of spaced-apart and intersectingelectrodes for applying an electric field to each optical elementtherebetween, wherein each optical element comprises electro-opticalmodulation material switchable into first and second response states inresponse to a control pulse of respective first and second polaritiesand a given amplitude and duration, the method comprising the stepsof:initially setting each optical element into the second responsestate; switching each optical element into a desired one of the firstand second response states; and thereafter holding each optical elementin the desired response state until the optical element is to beswitched again; wherein the steps of switching and holding comprisesequentially supplying a selection signal to one of the first set ofelectrodes while supplying a non-selection signal to other electrodes ofthe first set to select one set of optical elements and selectivelysupplying to each electrode of the second set of electrodes during thesupplying of the selection and non-selection signals one of first andsecond different response signals corresponding to the first and secondresponse states respectively to switch each optical element in theselected set to the desired response state, wherein the selectionsignal, non-selection signal, first response signal and second responsesignal comprise pulses configured in polarity, duration and amplitude toeffect as a difference therebetween an AC pulse train applied to eachoptical element which is symmetrical about a zero voltage axis withregard to total pulse amplitude and duration and includes, during thesupplying of the selection signal, a control pulse effective to switchthe optical element into the first response state when the firstresponse state is desired, and, during the supplying of thenon-selection signals, pulses which are ineffective to switch theoptical element from the desired response state.
 14. The methodaccording to claim 13, further comprising stabilizing the responsestates of the electro-optical modulation material by high frequency ACpulses on the pulse train, wherein the high frequency pulses have ahigher frequency than that of the control pulses.
 15. A method fordriving an electro-optical device having optical elements disposed atthe intersections between two sets of spaced-apart and intersectingelectrodes for applying an electric field to each optical elementtherebetween, wherein each optical element comprises electro-opticalmodulation material switchable into first and second response states inresponse to a control pulse of respective first and second polaritiesand a given amplitude and duration, the method comprising the stepsof:initially setting each optical element into the second responsestate; switching each optical element into a desired gradation of one ofthe first and second response states; and thereafter holding eachoptical element in the desired response state until the optical elementis to be switched again; wherein the steps of switching and holdingcomprise sequentially supplying a selection signal to one of the firstset of electrodes while supplying a non-selection signal to otherelectrodes of the first set to select one set of optical elements, andselectively supplying to each electrode of the second set of electrodesduring the supplying of the selection and non-selection signals one offirst and second different response signals corresponding to the firstand second response states respectively to switch each optical elementin the selected set to the desired response state, wherein the selectionsignal, non-selection signal, first response signal and second responsesignal comprise pulses configured in polarity, duration and amplitude toeffect as a difference therebetween an AC pulse train applied to eachoptical element which is symmetrical about a zero voltage axis withregard to total pulse amplitude and duration and includes, during thesupplying of the selection signal, control pulse having an adjustedeffective voltage value effective to switch the optical element into adesired gradation of the first response state when the first responsestate is desired, and, during the supplying of the non-selectionsignals, pulses which are ineffective to switch the optical element fromthe desired response state.
 16. The method according to claim 15,further comprising stabilizing the response states of theelectro-optical modulation material by high frequency AC pulses on thepulse train, wherein the high frequency pulses have a higher frequencythan that of the control pulses.
 17. In an electro-optical device havingoptical elements disposed at the intersections between two sets ofspaced-apart and intersecting electrodes for applying an electric fieldto each optical element therebetween, wherein each optical elementcomprises electro-optical modulation material switchable into first andsecond response states in response to a control pulse of respectivefirst and second polarities and a given amplitude and duration, themethod of driving the device comprising the steps of:switching eachoptical element into a desired one of the first and second responsestates; and thereafter holding each optical element in the desiredresponse state until the optical element is to be switched again;wherein the steps of switching and holding comprise sequentiallysupplying a selection signal to one of the first set of electrodes whilesupplying a non-selection signal to other electrodes of the first set toselect one set of optical elements, and selectively supplying to eachelectrode of the second set of electrodes in synchronization with thesupplying of the selection and non-selection signals one of first andsecond different response signals corresponding to the first and secondresponse states respectively to switch each optical element in theselected set to the desired response state, wherein the selectionsignal, non-selection signal, first response signal and second responsesignal comprise pulses configured in polarity, duration and amplitude toeffect as a difference therebetween an AC pulse train applied to eachoptical element which has an average voltage of zero and includes,during the supplying of the selection signal, a first control pulseeffective to switch the optical element into the desired response state,and, during the supplying of the non-selection signals, pulses which areineffective to switch the optical element from the desired responsestate.
 18. The method according to claim 17, wherein said pulse trainincludes, during the supplying of the selection signal, a precedingcontrol pulse having the opposite polarity of the first control pulse.19. The method according to claim 18, further comprising identicallymodulating the pulse shapes of the control pulses to effect gradation ofthe optical state of the optical elements.
 20. A method according toclaim 17, wherein the pulses of the pulse train during the supplying ofthe non-selection signals include voltage pulses having oppositepolarities and the same pulse shape.
 21. The method according to claim17, wherein the pulses of the pulse train during the supplying of thenon-selection signals have a frequency higher than those of said controlpulses.
 22. The method according to claim 17, further comprisinginitially setting the optical elements to one response state with apulse train having an average voltage value of zero.
 23. The methodaccording to claim 17, wherein the electro-optical modulation materialcomprises a ferroelectric liquid crystal.